U.S. patent number 8,940,267 [Application Number 13/536,502] was granted by the patent office on 2015-01-27 for method of purifying nanodiamond powder and purified nanodiamond powder.
This patent grant is currently assigned to The Arizona Board of Regents on Behalf of the University of Arizona, Canon Kabushiki Kaisha. The grantee listed for this patent is Palash Gangopadhyay, Jun Kato, Alexander Ashton Miles, Mamoru Miyawaki, Robert A. Norwood, Shabnam Virji-Khalfan. Invention is credited to Palash Gangopadhyay, Jun Kato, Alexander Ashton Miles, Mamoru Miyawaki, Robert A. Norwood, Shabnam Virji-Khalfan.
United States Patent |
8,940,267 |
Norwood , et al. |
January 27, 2015 |
Method of purifying nanodiamond powder and purified nanodiamond
powder
Abstract
A method of purifying a nanodiamond powder includes preparing
the nanodiamond powder, heating the nanodiamond powder at between
450.degree. C. and 470.degree. C. in an atmosphere including
oxygen, performing a hydrochloric acid treatment on the heated
nanodiamond powder, and performing a hydrofluoric acid treatment on
the nanodiamond powder obtained after performing the hydrochloric
acid treatment.
Inventors: |
Norwood; Robert A. (Tucson,
AZ), Gangopadhyay; Palash (Tucson, AZ), Miles; Alexander
Ashton (Tucson, AZ), Kato; Jun (Yokohama, JP),
Virji-Khalfan; Shabnam (Yorba Linda, CA), Miyawaki;
Mamoru (Tucson, AZ) |
Applicant: |
Name |
City |
State |
Country |
Type |
Norwood; Robert A.
Gangopadhyay; Palash
Miles; Alexander Ashton
Kato; Jun
Virji-Khalfan; Shabnam
Miyawaki; Mamoru |
Tucson
Tucson
Tucson
Yokohama
Yorba Linda
Tucson |
AZ
AZ
AZ
N/A
CA
AZ |
US
US
US
JP
US
US |
|
|
Assignee: |
The Arizona Board of Regents on
Behalf of the University of Arizona (Tucson, AZ)
Canon Kabushiki Kaisha (Tokyo, JP)
|
Family
ID: |
49778375 |
Appl.
No.: |
13/536,502 |
Filed: |
June 28, 2012 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20140004031 A1 |
Jan 2, 2014 |
|
Current U.S.
Class: |
423/446; 423/461;
977/775; 977/895; 423/460 |
Current CPC
Class: |
C30B
25/16 (20130101); C30B 25/165 (20130101); C30B
29/04 (20130101); C01B 32/28 (20170801); C30B
25/00 (20130101); B82Y 30/00 (20130101); B82Y
40/00 (20130101) |
Current International
Class: |
B01J
3/06 (20060101); C09C 1/56 (20060101); C01B
31/02 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
63-303806 |
|
Dec 1988 |
|
JP |
|
03271109 |
|
Dec 1991 |
|
JP |
|
2011-037693 |
|
Feb 2011 |
|
JP |
|
2446097 |
|
Sep 2010 |
|
RU |
|
2007/133765 |
|
Nov 2007 |
|
WO |
|
Other References
Petrov et al.; Detonation Nanodiamonds Simultaneously Purified and
Modified by Gas Treatment; Diamond & Related Materials; 16,
2098-2103; 2007. cited by examiner .
Shenderova et al.; Modification of Detonation Nanodiamonds by Heat
Treatment in Air; Diamond & Related Materials; 1799-1803; 2006.
cited by examiner .
V. Pichot et al., An efficient purification method for detonation
nanodiamonds, Diamond & Related Materials 17 (2008) 13-22.
cited by applicant .
Sebastian Osswald et al., Control of sp2/sp3 Carbon Ratio and
Surface Chemistry of Nanodiamond Powders by Selective Oxidation in
Air, J. Am. Chem. Soc. 2006, 128, 11635-11642. cited by applicant
.
Roberto Martin et al., Fenton-Treated Functionalized Diamond
Nanopartides as Gene Delivery System, www.acsnano.org vol. 4, No.
1, 65-74, 2010. cited by applicant .
Kristopher D. Behler et al., Nanodiamond-Polymer Composite Fibers
and Coatings, www.acsnano.org, vol. 3, No. 2, 363-369, 2009. cited
by applicant.
|
Primary Examiner: Gregorio; Guinever
Attorney, Agent or Firm: Canon U.S.A., Inc. IP Division
Claims
What is claimed is:
1. A method of purifying a nanodiamond powder comprising: preparing
the nanodiamond powder; heating the nanodiamond powder at between
450.degree. C. and 470.degree. C. in an atmosphere including oxygen
to reduce a content of sp.sup.2 carbon to less than 0.01 wt %;
after heating the nanodiamond powder to reduce the content of
sp.sup.2 carbon, performing a hydrochloric acid treatment on the
heated nanodiamond powder; and performing a hydrofluoric acid
treatment on the nanodiamond powder obtained after the hydrochloric
acid treatment, wherein a content of S (sulfur), Fe (iron), Al
(aluminum) and Si (silicon) is each reduced to less than 0.01 wt
%.
2. The purification method according to claim 1, further comprising
performing a density gradient separation process to separate the
nanodiamond powder according to one or more of particle size and
number of particle clusters.
3. The purification method according to claim 1, wherein the
nanodiamond powder is heated isothermally at 460.degree. C.
4. The purification method according to claim 1, wherein heating is
performed to remove sp.sup.2 carbon from the powder, the
hydrochloric acid treatment is performed to remove transition
metals from the powder, and the hydrofluoric acid treatment is
performed to remove aluminum and silicon from the powder.
5. The purification method according to claim 1, wherein performing
the hydrochloric acid treatment comprises treating with a first
acid solution having a first acid consisting of hydrochloric acid,
and wherein performing the hydrofluoric acid treatment comprises
treating with a second acid solution having a second acid
consisting of hydrofluoric acid.
6. The purification method according to claim 1, wherein heating
the nanodiamond powder in the atmosphere including oxygen comprises
at least one of: (i) heating in an atmosphere that is air, and (ii)
heating in an atmosphere where the oxygen is O.sub.2.
7. The purification method according to claim 1, comprising:
preparing a nanodiamond powder; removing sp2 carbon from the powder
by oxidization; removing transition metals by using a hydrochloric
acid treatment; and removing aluminum and silicon by using a
hydrofluoric acid treatment.
8. The purification method according to claim 1, wherein the
content of S, Fe, Al, and Si in the resulting purified nanodiamond
powder is each less than 0.01 wt % as determined by EDAX
spectra.
9. The purification method of claim 1, wherein the content of W
(tungsten), Ta (tantalum), Cr (chromium), Mn (manganese), Ag
(silver), Ca (calcium), Cu (copper), and Ti (titanium) in the
resulting purified nanodiamond powder is each less than 0.01 wt %,
as determined by EDAX spectra.
10. The purification method of claim 1, wherein, as determined by
EDAX spectra, peaks for carbon and oxygen are observed, and peaks
for S (sulfur), W (tungsten), Ta (tantalum), Fe (iron), Cr
(chromium), Mn (manganese), Al (aluminum), Ag (silver), Ca
(calcium), Cu (copper), Ti (titanium), and Si (silicon) are not
substantially observed.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of purifying nanodiamond
powder and highly-purified nanodiamond powder.
2. Description of the Related Art
Nanodiamonds are used as abradants because of their hardness, and
they can also be used as an insulating material, an optical
material, and a biomedical material.
In WO2007/133765, Gogotsi, et al. disclosed an oxidization process
to remove sp.sup.2 carbon from commercially available nanodiamond
powder. However, a small percentage of sp.sup.2 carbon can still be
detected after the oxidization process. In addition to the removal
of sp.sup.2 carbon, metal impurities might be an issue when the
nanodiamonds are used as an optical or biomedical material.
SUMMARY OF THE INVENTION
Embodiments of the present invention provide an apparatus and a
method for nanodiamond purification.
According to an aspect of the present invention, a purification
method of a nanodiamond powder is provided that includes preparing
the nanodiamond powder, heating the nanodiamond powder at between
450.degree. C. and 470.degree. C. in an atmosphere including
oxygen, performing a hydrochloric acid treatment to the heated
nanodiamond powder, and performing a hydrofluoric acid treatment to
the nanodiamond powder obtained after performing the hydrochloric
acid treatment.
According to another aspect of the present invention, a nanodiamond
powder is provided having sp.sup.3 carbon, wherein the content of S
(sulfur), Fe (iron), Al (aluminum), and Si (silicon) in the powder
is each less than 0.01 wt %.
Further features of the present invention will become apparent from
the following description of exemplary embodiments with reference
to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a thermogravimetric analysis (TGA) spectrum of a
nanodiamond powder before oxidation.
FIGS. 2A and 2B show electron energy loss spectroscopy (EELS)
spectra of a nanodiamond powder before and after heat treatment,
respectively.
FIGS. 3A, 3B, 3C, and 3D show energy dispersive x-ray spectroscopy
(EDAX) spectra of a nanodiamond powder at different steps in a
purification process, and each curve shows a different measurement
point in the same sample.
FIG. 4 shows EELS spectra of a nanodiamond powder at different
steps in a purification process.
FIG. 5 shows EELS maps recorded on nanodiamond powder after heat
treatment.
FIG. 6A shows blue and gray nanodiamond powders separated using
centrifugation.
FIG. 6B shows white, blue and gray nanodiamond powders separated
using a density gradient separation technique, along with their
transmission electron microscopy (TEM) images.
DESCRIPTION OF THE EMBODIMENTS
According to aspects of the invention, a purification method for
purifying nanodiamond powder can include the following three steps.
The first step is a heating step to oxidize carbon impurities
(sp.sup.2 carbon) in order to remove them from a nanodiamond
powder. The heating can be executed in an atmosphere including
oxygen. The atmosphere can be an air, or an oxygen gas. The
temperature of the heating can be selected from between 450.degree.
C. and 470.degree. C. For example, the powder can be heated at
460.degree. C. FIG. 1 shows the TGA spectrum of as-purchased
nanodiamond powder. The oxidation process onsets at about
460.degree. C. (indicated as a dashed line) in this spectrum, as
evident from the radiated thermal energy peak.
When a purchased nanodiamond powder substantially doesn't have
sp.sup.2 carbon before the first step, the heating step can be
omitted.
The second step is to remove metal impurities, especially
transition metals, from the nanodiamond powder after the
oxidization process. To remove the metal impurities, a hydrochloric
acid can be used. Hydrochloric acid may provide a high reaction
rate with the transition metals. The hydrochloric acid may be
provided as a solution of from 10% to 38.5% by weight of
hydrochloric acid, such as a solution of 37% by weight hydrochloric
acid. Hydrobromic and hydroiodic acids might also be used instead
of using the hydrochloric acid.
The third step is to remove other impurities, such as silicon,
silicon dioxide, aluminum, and aluminum silicate. To execute the
third step, a hydrofluoric acid can be used. The hydrofluoric acid
may be provided as a solution of from 4% to 48% by weight of
hydrofluoric acid, such as a solution of 47% by weight hydrofluoric
acid.
After the third step, a density gradient technique can be executed
in order to classify the nanodiamonds, optionally. The density
gradient technique allows for separation of the purified
nanodiamonds on the basis of density, which may provide separation
of nanodiamond materials according to particle size and/or number
of particle clusters. According to one embodiment of a density
gradient technique, a linear density gradient can be prepared using
60% OptiPrep.TM. (a commercially available density gradient
solution comprising a solution of iodixanol) in distilled water,
with relative densities of 60%, 50%, 40%, 30%, and 20%. One beaker
is prepared for each ratio and stirred for 15 minutes to ensure
good mixing. 14 ml of each in order of decreasing density are
carefully added to a 70 ml centrifuge tube via syringe. The
resulting structure is centrifuged at 3500 rpm for 20 minutes in
order to linearize the density within the tube. A sample of
centrifuged nanodiamond, possessing both a blue layer and a gray
bulk material, is agitated in deionized water and 1 gram sodium
cholate surfactant. The mixture is then sonicated and injected into
the linear density gradient which is centrifuged at 3500 rpm for 99
minutes. The small amount of material spreads out to form a large
cloud, and liquid is extracted from three regions within the
density gradient and centrifuged separately. The isolated solid is
collected and dissolved in a small amount of deionized water. Each
of the solids collected from the extracted regions of the density
gradient can be further classified according to one or more of
color, particle size and particle cluster size, among other
features, as discussed in further detail below.
Example 1
Nanodiamond powder can be produced via a detonation synthesis, and
also commercially obtained.
Step I: Isothermal Treatment
An example of an isothermal treatment procedure is as follows.
As-purchased nanodiamond powder (1.57 g) was placed in an oven at
460.degree. C. for 1 hr. After removing it from the oven, the
resulting powder was cooled down to room temperature to give
ND_Iso460-1hr as a light gray powder (1.43 g, 91% yield).
Isothermal cleaning at 460.degree. C. for approximately 1 hr
effectively removes sp.sup.3 carbon that is present in the
purchased powder. The level of sp.sup.3 carbon has primarily been
estimated by electron energy loss spectroscopy (EELS), where
spectral features are observed associated with sp.sup.3 carbon (at
.about.285 eV) and sp.sup.a carbon (at about .about.294 eV). The
EELS spectra can be recorded using a scanning transmission electron
microscope (JEOL 2010F), operated at 200 kV, equipped with TEM/STEM
and Gatan high-resolution GIFT EELS detectors.
As an example, FIGS. 2A and 2B show the EELS spectra for the
nanodiamond powder, before and after the isothermal treatment,
respectively. According to FIGS. 2A and 2B, it is observed that
sp.sup.2 carbon substantially doesn't exist in the powder after
isothermally heating the sp.sup.2 carbon. When a purchased
nanodiamond powder substantially doesn't have sp.sup.2 carbon
before the isothermal cleaning step, this step can be omitted.
After the isothermal treatment, the content of sp.sup.2 carbon was
found to be less than 0.01 wt %, as determined from high resolution
EELS maps obtained after isothermal treatment.
FIG. 5 shows EELS maps recorded on isothermally cleaned nanodiamond
powder. Using GATAN EELS analysis software the concentration of
sp.sup.3 C was calculated as 2042.+-.125 atoms/nm.sup.3, which
corresponds to 3.39.+-.0.21 g/cm.sup.3, comparable to diamond
density of 3.515 g/cm.sup.3. Similar measurements over the
reconstructed sp.sup.2 map over 284-290 eV yields a sp.sup.2
concentration 4.6.times.10.sup.-4 atoms/nm.sup.2 with residual map
density of .about.10.sup.-3 atoms/nm.sup.2. Assuming the residual
map density as the measurement noise level, it is concluded that
the sp.sup.2 C/sp.sup.3 C ratio is .about.5.times.10.sup.-7, which
corresponds to less than 0.01 wt % of sp.sup.2 carbon.
Step II: Hydrochloric Acid Treatment
An example of an HCl treatment procedure is as follows. The
nanodiamond powder (1.21 g) obtained after the isothermal treatment
was placed in 30 mL of 37% hydrochloric acid. The mixture was
heated to 120.degree. C. with stirring for 1 hr. After heating at
120.degree. C. for 1 hr, the resulting suspension was poured into
200 mL of deionized water. The suspension was allowed to settle
overnight to precipitate sufficient nanodiamond. The supernatant
was removed gently. The remaining precipitate was repeatedly rinsed
with deionized water until reaching the same pH as deionized water,
and dried overnight at 100.degree. C. in vacuo to give
ND_Iso460-1hr_HCl-1hr as a light gray powder (0.91 g, 75%
yield).
Step III: Hydrofluoric Acid Treatment
An example of an HF cleaning treatment procedure was performed as
follows. Nanodiamond powder (0.20 g) yielded from the above HCl
treatment was placed in 10 mL of 47-51% hydrofluoric acid. The
mixture was stirred at room temperature for 80 min before the
resulting suspension was poured into 200 mL of deionized water.
This suspension was allowed to settle for 3 hours to precipitate
sufficient nanodiamond. The supernatant was removed gently. The
remaining precipitate was repeatedly rinsed vigorously with
deionized water until reaching the same pH as deionized water, and
dried overnight at 100.degree. C. in vacuo to give
ND_Iso460-1hr_HC1-1hr_HF-80min as a light gray powder (0.15 g, 75%
yield). Plastic labware was used here instead of laboratory
glassware, due to the corrosive effects of HF.
Energy dispersive X-ray (EDAX) analysis is a technique that can be
performed with a scanning electron microscope and can provide
quantitative information on the elements present in a given field
of view. For each sample, eleven fields (50 .mu.m.times.50 .mu.m)
were analyzed. The EDAX spectra were observed with the Hitachi
S-3400 and S-4800. K.sub..alpha.* energies between 0 and 11 keV
were measured, which covers the majority of elements in the
periodic table combining L.alpha. and k.alpha. peaks.
FIG. 3A shows EDAX spectra of nanodiamonds as-purchased. The
horizontal line is energy in kiloelectron volts (keV), and the
vertical line (not shown) is relative intensity.
The primary peak positions of S (sulfur), W (tungsten), Ta
(tantalum), Fe (iron), Cr (chromium), Mn (manganese), Al
(aluminum), Ag (silver), Ca (calcium), Cu (copper), Ti (titanium),
Si (silicon), and Cl (chlorine) are 2.307 (S), 1.774 and 8.396 (W),
1.709 and 8.145 (Ta), 0.705 and 6.403 (Fe), 0.573 and 5.414 (Cr),
0.637 and 5.898 (Mn), 1.486 (Al), 2.984 (Ag), 0.341 and 3.691 (Ca),
0.930 and 8.040 (Cu), 0.452 and 4.510 (Ti), 1.740 (Si), 2.622 (Cl),
respectively.
FIGS. 3B through 3D show nanodiamond EDAX spectra after the each of
the purification steps, namely isothermal treatment, hydrochloric
acid treatment, and hydrofluoric acid treatment.
FIG. 3B shows the results after isothermal cleaning. There are only
minor changes in the EDAX spectrum after isothermal cleaning, since
no metals are presumed to be removed by this process.
The hydrochloric acid step (second step) removes transition metals
such as Fe, Cr, Ni, as well as other metals that readily form a
water soluble metal chloride. FIG. 3C shows EDAX spectra after HCl
cleaning, where it can be clearly seen that many of the metals
present in the as-received and isothermally cleaned samples in
FIGS. 3A and 3B are no longer present, in fact, almost all that
remains are Al, Si, S, Cl and small amounts of iron.
In the hydrofluoric acid treatment (third step) of the cleaning
process, the remaining non-carbon elements, primarily Al and Si,
are removed using hydrofluoric acid. FIG. 3D shows the EDAX spectra
after the HF cleaning step, and it can be seen that only carbon and
oxygen are left as impurities in the sample.
The above described three-step purification process was designed to
remove the carbon impurities (non-sp.sup.3 carbon) and inorganic
impurities (metal species, oxides, etc.) which are included in
commercially available nanodiamonds. The purification process may
include (1) isothermal treatment for removal of non-diamond carbon,
(2) hydrochloric acid treatment for removal of transition metals,
and (3) hydrofluoric acid treatment for removal of remaining
non-carbon species, including aluminum, and silicon.
Table 1 summarizes the quantitative EDAX data showing the wt % of
species in the nanodiamond powder during different stages of the
purifying process, including for the powder as-received, after
isothermal heat treatment, after hydrochloric acid treatment, and
after hydrofluoric acid treatment. The table shows that the
completely cleaned nanodiamond powder substantially does not have
any found metals (NF=not found), at the level of 0.01 wt %. The
level of 0.01 wt % is arrived at using a high-resolution leading
edge EDAX measurement system such as in S-4800 and JEOL 2010F,
which has resolution (lowest detection sensitivity) of few atomic
%, particularly for elements with atomic number Z>11.
Sensitivity of this technique increases with increasing atomic
number. Accordingly, as an array of elements with varied atomic
numbers are included, the detection limit (lowest detection
sensitivity) can be conservatively estimated as 0.01 wt %. This
data, together with the EELS data, shows that the major extrinsic
sources of color, namely sp.sup.2 carbon and metal impurities, have
been removed from the nanodiamond to at least the level of 0.01%,
and significantly lower in the case of sp.sup.2 carbon.
TABLE-US-00001 TABLE 1 Quantitative EDAX data (unit: wt %)
Isothermal As 460.degree. C., HCl HF Component Received 1 hr, Air
treatment treatment S 1.17-1.35 0.8-0.9 0.75-0.9 NF W 0.25 0.24 NF
NF Ta 0.18 0.19-0.2 NF NF Fe 0.1 0.15-0.2 <0.01 NF Cr 0.3-0.5
0.5-0.7 NF NF Mn 0.01 0.02 NF NF Al 0.01 0.08 0.1-0.12 NF Ag 0.16
<0.01 NF NF Ca 0.01 0.01-0.02 NF NF Cu 0.05 0.04 NF NF Ti 0.02
0.04 NF NF Si 0.15 0.2-0.25 0.2-0.35 NF Cl 1.1-1.2 1.2-1.3 0.05
NF
Example 2
In order to establish that the improved cleaning process does not
reintroduce sp.sup.2 carbon, a series of EELS measurements were
made on nanoparticles taken after each step by using a commercial
nanodiamond powder with little or no sp.sup.2 C, purchased from
Sigma-Aldrich. FIG. 4 shows that, other than overall signal level
variation, there is no change in the EELS spectrum during the
cleaning process. In FIG. 4, the horizontal line is energy in eV,
and the vertical line (not shown) is EEL signal count.
After HF cleaning and washing with water, the nanodiamond powder
was subjected to centrifugation, upon which the powder separated
into two distinct layers, blue on top of gray, as shown in FIG. 6A.
Under transmission electron microscopy (TEM) the blue layer was
found to contain smaller particles with the least number of
particle clusters. The gray was found to contain mostly clusters of
particles. Since nanoparticle clusters often exhibit increased
optical scattering, the blue layer having fewer clusters is of
interest for optical applications, such as in developing high
refractive index composites. Purified gray material can also be
used for other applications such as sensors or abrasive materials.
The blue layer contained smaller particles but also a large range
of diameters. To further size purify these particles the blue layer
was subjected to the density gradient separation technique, as
described above.
Example 3
By using the density gradient technique described above, the
nanodiamond powder can be classified as follows.
FIG. 6B shows the separated white, blue and gray nanodiamond
powders along with their transmission electron microscopy (TEM)
images. The white layer (collected from the top in the density
gradient method) contained particles with diameter 7.+-.2 nm, and
clusters with average diameter of 35.+-.10.2 nm. The blue layer
(collected from the bottom layer) contained particles with diameter
of 5.+-.4 nm and clusters of 102.+-.60.0 nm. The gray layer, which
is very similar to bulk, was found to contain particles with
diameter of 5.+-.7 nm and 220.+-.85.0 nm clusters. Thus, the
density gradient separation technique allows for the separation of
nanodiamond layers according to particle sizes and/or number of
particle clusters, such as to allow for isolation of layers having
reduced particle sizes and/or reduced numbers of particle clusters.
Of the separated layers, the nanodiamond particles recovered from
the white layer may provide characteristics suitable for optical
applications because of low optical scattering from the smaller
particles and the small number of clusters present in the white
layer. Particles from the blue layer can also be used for
wavelength specific optical applications such as luminescent
particles in biological tagging and as sensors. The particles
separated from the gray region are very similar to the ones
separated from the gray layer following centrifugation as described
above, and can be used for similar applications.
A cluster was defined as a collection of individual nanoparticles
that are attached to each other in the TEM images, a minimum number
of particles that were considered in this estimation of cluster
size was 2. TEM images recorded in 300 k magnification were used
and at this magnification TEM instrument has .about.0.6 nm
resolution. Using program ImageJ the pixels were calibrated for the
size based on the sale bar printed on each TEM image. A total of 5
sets of TEM images were used for each particle.
Nanodiamonds have been widely used in applications ranging among
abrasives, coatings, lubricants, polymer composites, electronic
devices, and the biomedical field. Nanodiamonds cleaned according
to aspects of the invention can work more effectively for these and
other applications than commercial nanodiamonds containing some
impurities. Aspects of the invention can provide a high efficiency
purification method, as shown above, which yields nanodiamonds that
are much purer than nanodiamonds produced by existing cleaning
processes.
Despite the removal of sp.sup.2 carbon and substantially all
measurable metal impurities in the cleaning process, the resulting
nanodiamond may remain a light gray color in some cases.
Accordingly, further separation of large particles using the
disclosed density gradient technique has been shown to be effective
in further reducing gray color.
As explained above, sp.sup.2 carbon and metal contaminations can be
removed to obtain purified nanodiamonds. The purified nanodiamonds
can fully demonstrate their bulk character with respect to
mechanical properties, chemical stability, optical properties (i.e.
high refractive index), electric properties, and thermal
conductivity. They can be used as materials for transparent resin,
glass, or plastic. For example, the purified nanodiamonds can be
used in a glass lens or a plastic lens. Since the content of the
metal impurities in the purified nanodiamonds is very low, such
purified nanodiamonds can also be used as biomedical materials.
While the embodiments according to the present invention have been
described with reference to exemplary embodiments, it is to be
understood that the present invention is not limited to the above
described embodiments. The scope of the following claims is to be
accorded the broadest interpretation so as to encompass all such
modifications and equivalent structures and functions.
* * * * *
References